Proving Units: The Tester That Tests the Tester (And Why GS38 Doesn't Quite Mandate One)

When a competent second-fix electrician isolates a circuit at the consumer unit, you'll see a small ritual that most homeowners never notice. They touch a tester to a small box from their tool pouch. Then they touch the tester to the conductors. Then they touch the small box again. The whole thing takes thirty seconds and looks like faff. It isn't. It's the prove-test-prove sequence required by HSE GS38, and the small box is the one piece of kit that stops a faulty tester from reading "dead" on a circuit that's still live. The small box is a proving unit. Most homeowners have never heard of one.

You almost certainly will not own a proving unit yourself. Knowing how the procedure works, though, lets you spot a sparky who's cutting corners on the one safety check that matters more than any other.

What a proving unit actually does

A proving unit is a small, battery-powered box that outputs a known AC voltage when you touch a tester's probes to its terminals. Cheap ones output a single voltage (typically 240V or 690V). Better ones step through a sequence (50V, 100V, 230V, 400V, 690V) so every LED on a multi-range voltage indicator lights up in turn.

The output is current-limited by design. The Di-Log PU690 specification, for instance, holds output current below 3.5mA at 690V into a 200kΩ load. That's around 2.4 watts. Plenty to drive the LEDs on a voltage tester. Nowhere near enough to cause a dangerous shock if you accidentally touch the probes. Proving units are safe to hold, safe to operate, and intentionally low-energy.

Why does that matter? Because the whole point of the device is that it lets the user generate a known live source on a workbench, in a van, or at the consumer unit cupboard, without having to find a real live circuit. A tester being checked against a working 230V socket is one option. A tester being checked against a proving unit is better. Here's why.

The prove-test-prove sequence

Every safe isolation procedure follows the same three steps. Skip the third and you've defeated the entire system.

The third step is the one people skip. It's also the one that saves lives. A two-pole indicator can fail mid-test for any number of reasons: a flat battery, a probe lead with a broken core, a blown internal fuse from a transient spike, an LED that fails after one too many shocks. Without the re-prove, you cannot tell the difference between a circuit that is genuinely dead and a tester that died while you were testing it.

What GS38 4th Edition actually says (and doesn't)

There's a widespread assumption that HSE GS38 mandates a dedicated proving unit. It doesn't, quite. The 4th Edition of GS38, the current HSE guidance for low-voltage test equipment, says the tester must be proved before and after use, "preferably on a voltage proving unit (some devices have built-in proving units) or otherwise on a known live source of similar voltage to the circuit under test (providing precautions are taken to prevent danger arising)."

Read that carefully. The proving unit is "preferred." A "known live source" is "permitted." Both routes satisfy GS38.

In practice, "known live source" means the live side of the consumer unit's main switch. The supply enters there from the meter and is always live unless the supply itself has been pulled. Touching the tester probes to the live and neutral on that side will light the 230V LED on a voltage indicator. That counts as proving.

So why bother with a dedicated proving unit at all? Three reasons that the trade has settled on by consensus.

First, the proving unit exercises every LED range. A 230V live source only lights the 230V LED. A multi-step proving unit (Megger MPU690, Kewtech KEWPROVE3) ramps through 50V, 100V, 230V, 400V, 690V. If the 400V LED on your tester has failed, the proving unit catches it. The live socket doesn't. That matters because the 400V indication is what you'll rely on if you ever encounter a three-phase supply (more common in larger properties than people think) or a fault that's left an unintended phase-to-phase voltage on a domestic conductor.

Second, a proving unit isn't affected by power cuts. If you've isolated a circuit, started the dead test, and the supply has a brief outage between isolation and re-prove, the "known live source" you were going to use for the re-prove is now itself dead. Your tester reads zero on the prove unit and zero on the dead circuit and you assume everything's fine. Five minutes later the supply comes back and so does the circuit you thought you'd proved dead. A battery-powered proving unit is independent of the mains supply and unaffected by this.

Third, a proving unit is always available. The dead test happens at the point of work, which might be a back bedroom or a loft. The "known live source" might be three floors away. The longer the trip from prove to test to re-prove, the more likely it is that the procedure gets shortened or skipped. A proving unit fits in a tool pouch.

For all three reasons, every competent UK electrician carries a proving unit even though GS38 doesn't strictly require one.

"Self-proving" testers are not the same thing

A voltage indicator that markets itself as "self-proving" is not the same as a tester used with an external proving unit, despite what some product listings imply.

The TIS 851 is the most common example. Its self-prove function works by detecting when the probes are briefly touched together: this triggers an internal LED test that confirms the display is working. That checks the indicator's own electronics. It does not apply real voltage through the probe leads, the front-end voltage detection circuit, or the full signal chain that does the actual measurement. Forum consensus among professional electricians is that the TIS 851 self-prove is "rather different to proving it at 230V". Adequate for some users, perhaps, but not equivalent to an external proving unit.

The Fluke T110 and T150 sit in a more interesting category. Their built-in self-test is more sophisticated and exercises more of the detection circuit, and is widely accepted by working electricians as adequate for daily proving. The Martindale VI-13800 takes a similar approach.

For a homeowner watching their electrician work, the practical takeaway is straightforward. If your sparky uses a self-proving indicator and that's all, ask which model. Fluke T110/T150 or similar is fine. A TIS 851 with no separate proving unit is a corner being cut. A separate proving unit alongside any indicator is the gold standard.

Models compared

There are essentially three classes of proving unit on the UK market: domestic-only (240V single-step), industry-standard (690V multi-step), and premium (multi-voltage AC/DC with extended features). Below are the units you'll see most often on a working van.

For most domestic and small commercial work, the Di-Log PU690 or Kewtech KEWPROVE3 hits the sweet spot. Both are 690V multi-step units that exercise every LED range on the tester and sit at the lower end of the standalone price range from the cheaper retailers. The Megger MPU690 is the same job with a slightly nicer build, a magnetic base for sticking to a consumer unit, and five discrete LED level indicators.

The Martindale PD240 is a different proposition. It outputs only 240V DC, and only enough to verify a tester rated for domestic work. It will not light the 400V or 690V LEDs on a multi-range indicator. If your tester is also domestic-only it pairs neatly. For anything beyond strictly single-phase 230V work, get a 690V unit instead.

The Fluke PRV240 is excellent kit but it's a brand premium. You're paying for the Fluke name and ruggedness. There's nothing it does that a KEWPROVE3 doesn't, at half the price.

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Buying as a kit

If you (or your electrician) needs both a tester and a proving unit, the kits are usually better value than buying separately. Three to know about:

• Martindale VT12PD240 kit. VT12 voltage tester plus PD240 proving unit plus carry case, in the typical kit price band. Domestic-focused. The PD240 limits its versatility but the VT12 is a solid GS38-compliant indicator.
• Di-Log DLPK6780 kit. DL6780 indicator plus PU690 proving unit at Screwfix, the cheapest of the three kits listed here. Better value, 690V output, and the DL6780 covers a wider voltage range.
• Kewtech KEWISO2 safe-isolation kit. KT1780 tester plus KEWPROVE3 plus Kewlok padlock plus case, priced at the upper end of the kit band. The complete safe-isolation package including the lockout for the MCB. Pick this one if your electrician is starting fresh and needs the full kit in a single purchase.

Why CAT ratings matter less here than you'd think

Voltage testers carry CAT ratings (CAT II, III, IV) that describe the fault energy they can survive at different points in an installation. A proving unit is rated CAT II 750V on most domestic models, for example the Di-Log PU690.

That sounds low for a tool used in safe isolation, but it isn't a problem. The proving unit isn't connected to the installation under test. It's a self-contained, battery-powered, current-limited source sitting on a workbench or in a tool pouch. The CAT rating reflects transient protection on the proving unit's own circuit. The installation category for the dead test itself is determined by the indicator (which should be CAT III 600V minimum for consumer-unit work, CAT IV 600V for the supply origin).

In short: don't worry about CAT ratings on a proving unit. Worry about them on the indicator.

The flat-battery trap

Every battery-powered proving unit has the same theoretical weakness as a battery-powered tester: a flat battery means no proving. The unit can fail to output voltage and fail to indicate that it's failing.

This matters because the proving unit is supposed to be the device that catches a tester problem. If the proving unit is itself broken in the same way, the prove-test-prove sequence collapses to "test." You think you proved the tester. You didn't. The tester might still be faulty. The circuit might still be live.

Two practical defences. First, every common proving unit has a low-battery LED indicator. Check it lights green (or whatever the manufacturer's convention is) before you trust the prove. The Megger MPU690 and Di-Log PU690 both have explicit power LEDs separate from the voltage output indicator. Second, replace the AA cells annually as a matter of routine, not when the low-battery LED comes on.

A small minority of professional electricians use battery-free neon proving testers (Drummond units are the well-known example) precisely to eliminate this failure mode. They're rarer because they're less convenient, since they need a real live source to operate against, but they remove the dead-battery risk entirely.

Do you need to own one?

For the overwhelming majority of homeowners managing an extension build: no. A proving unit is a tool your electrician brings. Your job is to confirm they're using it properly, not to do the safe isolation yourself.

The legal position reinforces this. Adding new circuits, rewiring, or working in special locations (kitchens, bathrooms, gardens) is notifiable work under BS 7671 and Part P of the Building Regulations. It must be done by a registered electrician or signed off by Building Control. A homeowner doing this work themselves without certification is committing a Building Regulations breach that will surface at sale or remortgage.

What homeowners can legally do is non-notifiable: replacing a faceplate like-for-like, swapping a light fitting, changing a fused spur. Even these tasks demand safe isolation. For one-off jobs of that scale, a self-proving two-pole tester (TIS 851 or similar at the budget end, or a Fluke T110 at the higher end) covers the proving requirement without needing a separate unit. GS38 is fine with that. Buy a tester, prove against the live side of the consumer unit's main switch as a fallback, and accept that you're operating at the simpler end of the safety procedure.

The single tool a homeowner managing a build should own is a two-pole voltage indicator, not a proving unit. See the voltage tester guide for what to buy and why. Independent verification that a circuit your builder claims is dead is genuinely dead is worth far more than the modest cost of the tester.

What to ask your electrician

You don't need to be an electrician to verify your electrician is following GS38. Three questions cover it.

1. "What proving unit do you carry?" The right answer names a specific model, like KEWPROVE3, MPU690, or PU690, or describes a self-proving tester that you can later look up (Fluke T110, T150). A blank look or "I just use a live socket" is a flag. GS38 permits the live socket but the trade has moved past it.

2. "Will you prove the tester before and after the dead test?" The answer should be "yes, every time." If they hesitate or say "before is enough," they don't understand the procedure. The post-test re-prove is the part that catches a tester that failed during the test.

3. "Can I watch the safe isolation when you do the consumer unit work?" Any competent electrician will say yes without thinking about it. Watch what they do. You should see them touch the proving unit, touch the conductors, touch the proving unit again. Three steps. If they only do two, you've spotted a problem.

Related electrical-test kit

The proving unit is one component of the safe-isolation toolkit. The companion pieces:

• Two-pole voltage indicator. The actual tester. Touches the conductors and reads voltage. See the voltage tester guide for buying advice and CAT-rating explanation.
• MCB lockout device. Physically prevents the breaker being switched back on while work is in progress. A strip of insulating tape is not acceptable under Best Practice Guide 2. See the MCB lockout device page for models.
• Padlock with a single key. Held by the person doing the work. The whole point is that nobody else can re-energise the circuit.
• Warning tag. Identifies who isolated the circuit, when, and why.

A complete safe-isolation kit (tester, proving unit, lockout, padlock, tags) sits in the upper part of the typical kit band: see £135 – £150 for the comparable Di-Log kit pricing. The Kewtech KEWISO2 is the commonly recommended starter kit and includes everything except the warning tags.

Where you'll need this

• Safe isolation and proving dead - the full procedure that uses the proving unit at two points in the sequence
• First-fix electrics - your electrician will isolate and prove dead before connecting new cables to existing circuits
• Second-fix electrics - safe isolation at the consumer unit before connecting faceplates, switches, and the cooker hob

Proving units appear at every stage of any extension or renovation project that involves working on existing electrical circuits. The procedure is identical whether the work is a single faceplate change or a full rewire.

Safety notes

The single most important thing a homeowner can do on an extension build is verify the electrician is competent and follows safe isolation properly. Watching the prove-test-prove sequence at the consumer unit takes thirty seconds. It's the cheapest quality control you'll do on the entire project.